Removing unwanted surface contaminants and staining, such as rust or scale, from metal objects is a ubiquitous and at times very vexing problem that plagues a huge array of domestic, commercial and industrial installations, products and systems.
Not surprisingly, acids are an essential component in the myriad of cleaning products that exist and can be used in the extensive number of cleaning applications that require their unique cleaning and solubilizing attributes. One of the most common applications of acid-based cleaning solutions is found in the areas of descaling and metal oxide (e.g. rust) removal. The use of the word scale herein will generally refer to insoluble salts such as but not limited to calcium carbonate, sulfates and aluminosilicates. Metal oxides would encompass water insoluble oxides of metals such as but not limited to transition element oxides.
The formation, accumulation or build-up of scale can be a significant problem in many residential, commercial and industrial applications as it can diminish or even completely stop fluid flow through pipes, reduce heat exchange efficiency in boilers and evaporators, and adversely affect both utility and aesthetics in commercial and domestic situations such as in bathroom showers and fixtures (among many other instances). Calcium carbonate is a very commonly encountered scale in domestic, institutional and industrial applications. While removing the offending rust or scale is the primary objective, an equally important goal in many applications is to remove the rust or scale without changing the color of, or damaging, the underlying metal.
Theoretically, any acid that produces a calcium salt that is soluble in water/acid may be used to eliminate calcium carbonate scales. Mineral acids such as hydrochloric acid and phosphoric acid have been most often used for this type of scale, but such acids also present significantly elevated health, safety and environmental issues. For example, concentrated hydrochloric acid, both in solution (and the acidic mist that often accompanies use of the solution) can have a corrosive effect on human tissue, and can also cause damage to the eyes and lungs (among other adverse effects). See, for example, http://www.mdguidelines.com/toxic-effects-hydrochloric-acid, and https://medlineplus.gov/ency/article/002498.htm for the descriptions and warning about the use of hydrochloric acid. The latter website states: “Hydrocholoric acid is a lear, poisonous liquid. It is highly corrosive, which means it immediately causes severe damage, such as burning, on contact.” That website goes on to describe the many adverse reactions and symptoms that can resulting from touching, swallowing or even breathing the fumes of, hydrochloric acid, and how immediate treatment is required. Therefore, products containing hydrochloric acid must be used very carefully, and it is recommended that the person using such products first don personal protective equipment, such a gloves, googles, chemical-resistant clothing and shoes. These health and safety concerns have caused the United States Environments Protection Agency to rate and regulate hydrochloric acid as a “toxic” or “hazardous” substance.
The adverse environmental effects of hydrochloric acid are also significant, and are becoming more of a concern as they are increasingly recognized. See, for example, the article entitled “Hydrochloric acid: an overlooked driver of environmental change” that was published on Mar. 1, 2011 by the US National Library of Medicine, National Institute of Health (https://www.ncbi.nlm.nih.gov/pubmed/21288016), and the article entitled “HCI's Overlooked Environmental Effects” that was published on Feb. 3, 2011 by Chemical and Engineering News. (http://pubs.acs.org/cen/news/89/i06/8906scene.html?T+Online+News).
In addition to these safety concerns for the user and environment issues, the storage, shipping and importation of such substances is regulated and require special handling and equipment, which can add materially to production and delivery costs associated with this acids. Therefore, notwithstanding its efficacy, the use of hydrochloric acid is often not the preferred option, such that searching for and developing alternatives that are as or nearly as effective as this “toxic” chemicals without the concomitant health, safety, environmental and other risks, has been is an ongoing endeavor in the field.
In the search for alternatives, organic acids such as citric, lactic and glycolic acid have also been employed in cleaning solutions as they are the more environmentally friendly and safer options for calcium carbonate scale removal. However, these acids are not as effective as hydrochloric acid (for example, the rate of scale removal using these acids is approximately 1/20th that of hydrochloric acid; and exhibit a lower solubility of the calcium salts). These other acids may also contribute significantly to undesired Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) in industrial effluents, and this can limit their useful in these markets as well.
Urea acid salts have also been shown to have excellent application in this regard as they have much better health and safety profiles as compared to mineral acids (non-corrosive to skin, non-fuming, and lower corrosivity to metals). Further, the inherent acid strength in urea acid salts is significantly higher than organic acids (0.2 pKa vs 4 pKa) and therefore the rate of reaction with calcium carbonate scale to produce carbon dioxide and the associated calcium salt is much more rapid. In addition, the contribution of urea acid salts to BOD and COD can be significantly lower than with the organic acids (approximately one-third to one-half as much). Therefore, urea acid salts have shown some utility in this area.
The most commonly used urea acid salts available commercially are urea hydrochloride and urea sulfate. The latter compound has limited use in calcium carbonate removal because of the low solubility of calcium sulfate that is formed during use. Nevertheless, these chemistries have been shown to have excellent utility in multiple applications and in a wide range of applications, from retail-based bathroom cleaners to large scale industrial descaling to oil-well stimulation.
Another prevalent type of scale that is often encountered and must be removed includes the silicates, as well as silica (SiO2, which is the common ingredient in sand). Simple silicates such as calcium silicate exist where the silicate is anionic and the calcium is the counter cation. The structure of the molecule is fairly simple and as such, it is a usually a relatively simple task to solubilize it. Many of the mineral acids listed above can be effective in this regard.
Calcium silicate, however, is not a commonly encountered scale. None of the acids discussed above are as effective as desired in the removal of many common silicates where there may be combinations of Ca, Mg, Al, Fe (or other elements) as the cations, such that dissolution with most mineral acids is not possible. The structure of these silicate compounds can be quite complex. The mineral acids are also ineffective at solubilizing silica (silicon dioxide). Similarly, certain metal oxides such as but not limited to aluminum oxide, are extremely difficult to solubilize using the mineral acids described above. For example, it has been found that solubilizing aluminum oxide from aluminum rails on trucks is a very difficult task with limited practical options available.
It is generally understood that hydrofluoric acid (that is, hydrogen fluoride, chemical symbol and hereinafter referred to as “HF”) has proven to be a very effective chemistry of those previously available to solve these type problems as to which the other acid-based cleaning solutions mentioned above have not been found to be adequately effective in most instances. For example, in the case of complex silicates, HF reacts with the silicon center of the molecule and produces hexafluorosilicic acid which is highly soluble and which also forms soluble salts with calcium ions, which otherwise would precipitate as calcium fluoride. Thus, the combination of free hydrogen fluoride and the hexafluorosilicic acid effectively solubilizes the entire scale formation quickly.
Additionally, aluminum oxide which forms naturally on aluminum surfaces that are simply exposed to air, is solubilized by HF almost immediately upon contact at quite low concentrations, such as 0.1% active solution. It is believed that the mechanism of this solubilization is the formation of the metal fluoride (AlF3 or ALF4(1-)) in this case. Unfortunately, once the oxide layer is solubilized, the HF immediately attacks the underlying bare elemental aluminum, and usually in conjunction with sulfuric acid, can form a bright white surface, with the term “aluminum brightening” being commonplace in describing this application. In some applications, brightening may be desired, but there are other applications where that is unwanted. Therefore, in addition to the inherent safety, health and environmental issues attendant with the use of HF, the aluminum brightening after-effect is also a drawback in many applications. There may also be unwanted physical etching of the surface metal. Therefore, even without the health, safety and environmental concerns, HF is not the preferred choice in many applications.
Heretofore, HF has been generally considered by many in the industry as the most efficient chemistry to deal with the removal of complex silicate scales and metal oxides such as aluminum oxide. However, there are many other considerations that render the use of HF implausible or certainly undesirable. First and foremost is the health and safety profile of HF. Specifically, HF is able to penetrate through human skin on contact. It is has been postulated that the relative weakness of the HF acid (pKa 3.2) plays a significant role in this phenomenon. If it were a stronger acid, then it would exist in a more dissociated and ionic state, which would most likely impede passage across the skin. Exposure to HF at lower concentrations of 1-5%, if untreated, will result in burns to human skin that are not immediately evident, so the person being harmed may not be immediately aware of the danger and injury being sustained, such that prolonged exposure, and hence greater injury, may result. Eventually, however, tremendous pain develops in the area around the contact point for the victim. Often, necrosis of the skin will become evident and blackening of the affected finger/toe nails will take place, even after subsequent medical attention is received, frequently involving calcium gluconate injections. Therefore, the use of HF is often not preferred, notwithstanding its efficacy in a given application.
While the health, safety and environmental concerns with some acids can be ameliorated to a degree by combination with urea, that is not the case with HF. The relatively weak strength of HF as an acid is a reason why it will not effectively form a salt when mixed with urea. It may be that HF is simply not strong enough to react with urea to form a salt. This precludes one potential mechanism for making a safer version of HF.
In the case of higher concentrations of HF (when handling the raw material at 48% and higher content), unless immediate treatment with calcium gluconate cream is given, far more serious health effects due to hypocalcaemia may result in significant tissue damage requiring amputation, and even death may occur. Thus, as is well known, HF is a very “toxic” substance, which must be handled with extreme caution. And, as mentioned above, the significant adverse environmental effects of HF have become much more of a concern.
A further complication with the use of HF is the non-discriminatory nature of the corrosivity of the product. As an example, a silicate or silica based scale may coat a pipe or metal alloy in a boiler or evaporator, severely restricting the flow of fluids or impacting heat exchange efficiencies. HF would be able to remove the scale, but would also likely also corrode the underlying metal. And because the accumulation of scale is rarely if ever uniform on the underlying metal substrate, the HF will often contact and corrode the metal substrate in some areas while still dissolving the scale in other areas. The resulting corrosion to the underlying metal may be so damaging as to preclude the use of HF altogether. Another problem encountered with HF is when it makes contact with glass surfaces, either intentionally or inadvertently (such as may occur in an overspray situation). Depending upon the amount and solution strength, opacification of the glass can be virtually immediate, causing the need to replace the glass.
A similar problem can occur in cleaning transportation vehicles where the rails on the trailers, fuel tanks and other aluminum parts on the truck become dull with exposure to oxygen (causing the formation of aluminum oxide which creates a cloudy or dull surface appearance). Treatment with HF, typically formulated with sulfuric acid for this application, will effectively remove the dull surface material, but then almost immediately results in an oxidative attack of the base aluminum. This is evidenced by the evolution of hydrogen gas which is seen as effervescence on the metal surface. The result is a bright white surface that is generally viewed as undesirable, but has come to be grudgingly accepted as preferable to the more unsightly dull appearance before treatment. Most users, however, would prefer the bright shine of aluminum with only oxide removed. Therefore, although HF has proven effective in the removal of aluminum oxide, it has serious drawbacks as well.
Ammonium bifluoride (ABF) is a chemistry which is frequently used in the industry as an alternative to HF, mainly in an attempt to overcome some of the health and safety issues associated with the use of HF. ABF is a solid and is safer to use than the 48% or 70% liquid HF because the solid ABF will not cross the skin barrier. However, if there is any moisture (sweat) the user's skin, which is the case in many application situations, the ABF will immediately react with the sweat to form HF (0.5 mole for every mole of ABF) which can cause the severe health effects mentioned above. Further, when ABF is in aqueous solution (which is also common in many application situations), HF is formed and will have all of the health, safety and corrosivity issues associated with that chemistry. Therefore, while ABF is a preferred chemistry for some applications, it still has drawbacks that do not allow it to perform as an effective replacement for HF in many applications.
Thus, there exists a need in the art for chemistries that are able to demonstrate some or all of the functionality and efficacy of HF without its inherent and negative acute and chronic health, safety, environmental, corrosivity and other problems and drawbacks.
One alternative that has proven effective and safe, and which provided a significant advance in this field, is a chemistry based upon urea tetrafluoroborate (hereinafter “uTFB”). See, for example, U.S. Pat. No. 8,796,195. As described in that patent, the addition of low concentrations of acid inhibitors produced a chemistry that was a non-irritant to skin upon contact with the “as supplied material” having the total acidity typically associated with 48% fluoboric acid. Further, the uTFB chemistry has shown to be non-corrosive to mild steel, thereby allowing for non-regulated ground transport of the material in the U.S., which results in reduced shipping costs. Additionally, the corrosivity of the uTFB product has proven to be very low on a variety of metals such that commercially acceptable descaling operations that were not otherwise possible by chemical means, particularly with HF, can be effectively undertaken with the uTFB chemistry. Therefore, this uTFB chemistry has proven to be effective in multiple applications involving the removal of certain complex silicates, silica and aluminum oxide removal.
For example, polished aluminum wheels are commonly used on long- and short-haul trucks and trailers, due to the strength and light weight of the aluminum used. These wheels are constantly exposed to sunlight, oxygen, road grime and other contaminants. Therefore, they require frequent cleaning. While HF would be a very effective solution and was often used, in practice today it is very rarely used on these wheels due to both the pitting/corrosion of the metal and the permanent whitening of the metal that the HF will cause. The uTFB chemistry was an effective substitute because it is not only effective at removing the aluminum oxide thus exposing the polished aluminum below, but also is very slow to attack the bare aluminum and burn or whiten the metal. Therefore, it provided a safe-to-use chemistry that effectively cleaned and removed aluminum oxide without damaging the base metal.
Notwithstanding these substantial benefits and improvements in many applications realized with the uTFB chemistry, it is not a superior solution in some applications.
One of the highest volume cleaning applications in which this is encountered is in regard to cleaning the aluminum rails found on most transportation vehicle trailers. On new rails that have not been previously cleaned with HF, the uTFB chemistry will be quite effective in cleaning and brightening this metal, usually requiring a solution of about 1% activity. Unfortunately, the majority of the rails currently in use were previously routinely treated with HF, which, as described above, the HF caused significant etching of the surface and permanent whitening. Such surfaces are not well suited for cleaning with the uTFB chemistry, unless significantly higher concentrations (5-10%) are employed. Although even at this higher concentration, the safety profile of the uTFB chemistry is much better than for HF, the price-per-application of the higher-concentration uTFB material can be cost-prohibitive for some end users. Therefore, the uTFB chemistry was primarily used in this application as the “safe version” aluminum brightener that typically represents around 2-5% of total aluminum brightener applications.
It has also been found that the uTFB chemistry is not satisfactorily effective for the removal of rust and iron oxides in some circumstances. In those instances, supplemental chemistry must typically be added in order to improve efficacy in this application to an acceptable level. However, this adds costs and other drawbacks. Therefore, a need in the art existed for a chemical technology that does not use HF or require supplemental chemistry to be added to uTFB in order to improve iron oxide removal, and for other applications.
Another area in which these type cleaning chemistries have found particular utility is with complex silicates, where the uTFB chemistry has proven to be effective in some circumstances and applications where standard mineral acids would be ineffective. However, the uTFB chemistry has also been found to be relatively ineffective with other complex silicate scales. Thus, the uTFB chemistry is not a universal solution for this cleaning application.
In examining why the uTFB chemistry is not universally effective in this regard, one interesting feature of the uTFB chemistry that has been observed is the formation of some hexafluorosilicic acid during the dissolution of aluminum silicate. This appears to at least partly explain the ability of the uTFB chemistry to solubilize these difficult silicates. However, this phenomenon is difficult to understand in some ways, since the kinetics are not favorable for transfer of fluorine from boron to silicon (the B—F bond being much stronger than the Si—F bond). This is perhaps explained by the fact that one way to synthesize fluoboric acid is to react boric acid with hexafluorosilicic acid. This results in the formation of tetrafluoroboric (fluoboric) acid and SiO2, however, without the significant exothermic reaction that is seen with the reaction of HF and boric acid. Nonetheless, this is important empirical information that has aided in the development of additional chemical technologies to overcome some of the technical and commercial issues and drawbacks described above.
In addition to the cleaning applications discussed above, there are innumerable other applications in which an acid solution would be the preferred chemistry to solubilize the particular scale, metal oxide or other contaminant from a metal or other substrate, but an HF solution is not preferred apart from health, safety and environmental issues. This could be because the HF will damage the metal or other substrate or the paint or other material on the substrate, or because of safety concerns. Very often, an acid cleaning solution is used by persons who have not been properly trained in the proper use and storage of toxic materials, and even if the proper instructions are included with the product, will either ignore them, or will not have the safety apparel readily available for safe use. A few such examples, without limitation, include cleaning road and atmospheric residue and grime from auto and truck bodies without damaging either the underlying metal or the paint applied on it; cleaning vehicle motors and motor parts, cleaning rusted tools, cleaning metal landscaping equipment, cleaning common household wares and appliances made of metal, and many more applications.
It is accurate to state that a long felt need in the art, the answer to which has proven quite elusive, is the development of a cleaning chemistry that can effectively remove scale and rust (and other residue and contaminants) from a wide variety of metal and other substrates, that can do so without damaging or undesirably “brightening” the substrate or damaging surface materials such as paint, or that will cause glass to become opaque, that can do so at sufficiently low acidic concentrations so the chemistry is not harmful to humans and the environment, and is not considered a “toxic” material that is subject to government regulations and concomitant higher costs in shipping, storage and use.